This invention relates to hoists for raising and lowering the dump beds of trucks. More particularly, this invention relates to dual acting truck hoists which exhibit increased lifting efficiency and safety.
Extendible and retractable hoists placed between the frame of a vehicle and its dump bed (body) for raising and lowering the dump bed about a pivoting hinge are well known in the art and come in many different types and designs, whether they be rear or side dump vehicles. The term xe2x80x9ctruckxe2x80x9d is used herein to describe all types of vehicles which employ such hoists, including light, medium and heavy duty dump trucks and trailers.
In a typical hydraulic truck hoist, one or more hydraulic cylinders lie lengthwise between the frame rails of the vehicle and underneath the truck bed. As a hoist""s cylinder extends, one end of the hoist pushes against the underside of the dump bed while the other end of the hoist pushes against the frame of the truck to which it is pivotally attached. This causes the bed to rotate about its rear hinge pivot whereby the dump bed is raised. Retraction of the cylinder causes the bed to be lowered.
In many designs, the hoist cylinder is nearly parallel to the dump bed when in its lowered position, and thus has very little initial leverage at the beginning of the lifting process. During this initial lift period most of the force of the cylinder is directed towards the rear hinge rather than in the more efficient raising direction, which changes as the body is raised. Due to this inefficiency, larger cylinders are used than would otherwise be required. As is rather apparent, the size of the cylinder can not be changed during lifting and thus an oversized cylinder(s) to accommodate the initial inefficient lifting process must be used.
Many types of hydraulic hoists have been developed to address this initial lift problem. In one example, telescopic cylinders are mounted farther forward on the truck frame. This provides the cylinder with greater initial leverage. However, the cylinder in such devices usually must have a rather long stroke to achieve a sufficient dump angle (i.e. height of lift of the dump bed), which is a known disadvantage in the art.
Other attempts to improve these hoists have included the use of a simple lever arm to provide the lifting motion in an attempt to obtain a more efficient initial lift angle. Examples include those hoists disclosed in U.S. Pat. No. 4,052,105. Other designs include those disclosed in U.S. Pat. Nos. 2,509,911; 2,603,518; and 4,762,370. These patents disclose hoist designs in which a lever arm aids the cylinder during initial lift of the truck bed, and thereafter the cylinder completes the lifting process. Such systems are sometimes referred to as xe2x80x9cdual actingxe2x80x9d systems, and the lever arms may be referred to as xe2x80x9cboosterxe2x80x9d arms. This invention adopts in its preferred embodiments this xe2x80x9cdual actingxe2x80x9d concept and, as described below, improves upon former designs in this regard.
Another problem faced in the dump truck art is the potential for operator abuse of the system if a portion of the load being carried in the dump bed (e.g. wet dirt) xe2x80x9changs upxe2x80x9d in the bed during the dumping operation and thus is not dumped as intended. In such situations, it is not unusual for the truck operator, with the bed fully raised, to put the transmission in gear, accelerate the engine and quickly xe2x80x9cpopxe2x80x9d the clutch, causing the truck to lurch, usually forward. As the truck lurches, the operator then quickly steps forcefully on the brake pedal. As the truck lurches, various parts of the hoist, as well as the dump bed, are pulled away from their established positions with respect to each other. Then, when the brakes are suddenly applied, the parts under force added to by the heavy dump bed, rebound back against those elements in the system which established the original position. The maneuver itself is designed to use the jarring effect of the rebound to dislodge the hungup portion of the load in the bed, and it is not unusual for the maneuver to be repeated several times to insure the intended result. Significant damage to various truck hoist parts, the hydraulic system, the truck, and the dump bed itself can result from this practice. A hoist which prevents or discourages this practice, or at least mitigates the potential for damage if the maneuver is conducted, is clearly needed in this art, and it is an object of this invention in certain preferred embodiments to provide such a hoist design.
In view of the above, it is apparent that there exists a need in the art for a hoist which overcomes, mitigates, or solves the above problems in the art. It is a purpose of this invention to fulfill this and other needs in the art which will become more apparent to the skilled artisan once given the following disclosure.
Generally speaking this invention fulfills the above needs in the art by providing:
a hoist for raising and lowering a truck bed to and from a truck frame, the hoist comprising:
a first extendible and retractable cylinder having a first end and a second end;
means for attaching the first end of the first cylinder to a truck frame;
a second extendible and retractable cylinder having a first end and a second end;
means for attaching the first end of the second cylinder to a truck frame, and means for attaching the second end of the second cylinder to a truck bed;
a lever mechanism comprising a base member attachable to a truck frame, an elongated arm having a first end and a second end, a first pivot connecting the second end of the first cylinder to the first end of the elongated arm, a second pivot connecting the base member to the elongated arm at a location intermediate the first and second ends of the elongated arm, means located proximal the second end of the elongated arm for contacting a member of the truck bed, a stop mechanism for limiting the extent of rotation of the elongated arm about the second pivot;
wherein extension of the first and second cylinders, when the hoist is installed on a truck, elevates the truck bed to a first elevated position; and
wherein when the first cylinder extends to a predetermined length, extension of the second cylinder thereafter disengages the means proximal the second end of the elongated arm from the member of the truck bed, and elevates the truck bed to a second elevated position which is a greater elevation than the first elevated position.
In a preferred embodiment of this invention, the cylinders employed are telescopic cylinders comprised of a base cylinder and at least one tubular smaller cylinder or rod extending from the base cylinder, wherein the first end of the smaller rod is connected to the first pivot and its opposite second end is connected to the next largest cylinder or base cylinder such that the base cylinder is connected to the truck bed. It is understood, of course, that while both a single stage cylinder and multi-stage, telescopic cylinder are illustrated, either cylinder type, where appropriate, is contemplated for use therein.
In further preferred embodiments, the connection of the tubular cylinder to the first pivot comprises the conventional crosshead member of a hydraulic cylinder.
In a still further preferred embodiment of this invention, the first pivot is offset from the second pivot in a manner described below thereby to achieve certain unique results.
This invention will now be described with respect to certain embodiments thereof as illustrated in the following drawings, wherein: